1. Field of the Invention
The present invention relates to a microstructure and a manufacturing method thereof.
2. Description of the Related Art
Research has been conducted on manufacturing micromachines by using a process for manufacturing a semiconductor element or a method similar to that process. For example, a three-dimensional structure having a space portion and a portion moving in the space portion, such as a gear or a bridge structure has been manufactured by processing a silicon wafer.
In recent years, research has been advanced on manufacturing a micro electromechanical device having one function by manufacturing a structure as described above and a semiconductor element over the same substrate or by attaching them manufactured separately, to each other. Such a micro electromechanical device is called a MEMS (Micro Electro Mechanical System), a micromachine, a micromachining, or the like.
Micro electromechanical devices (micromachines) are roughly classified into two groups according to their manufacturing methods. One is called a bulk micromachine which is obtained by manufacturing a three-dimensional structure in such a way that a silicon wafer or an SOI (Silicon On Insulator) substrate itself is processed by etching or polishing. The other is called a surface micromachine which is obtained by manufacturing a three-dimensional structure in such a way that a thin film is stacked over a substrate such as a silicon wafer and the thin film is processed by photolithography and etching.
In the case of the bulk micromachine where the substrate is processed from various directions, the degree of freedom in manufacturing a three-dimensional structure is so high that various structures can be manufactured. However, many steps thereof are different from those in a process for manufacturing a semiconductor element. Therefore, in many cases, a semiconductor element and a structure are separately manufactured and then attached to each other, or they are packaged into one without being attached to each other. In the case of the surface micromachine, a general process for manufacturing a semiconductor element is applied.
Here, a basic structure and a manufacturing method of a structure in a surface micromachine will be described. First, as shown in FIG. 12A, a sacrifice layer 1002 is formed to form a space portion of a structure. This sacrifice layer 1002 is removed later. Note that although the sacrifice layer 1002 is formed over a substrate, the substrate is not shown in the drawing.
Next, as shown in FIG. 12B, a structural layer 1004 is formed over the sacrifice layer 1002. The structural layer 1004 is formed over the sacrifice layer 1002 so that the structural layer 1004 has a three-dimensional shape.
Next, as shown in FIG. 12C, the sacrifice layer 1002 is etched away. A space portion 1006 is formed under the structural layer 1004 by removing the sacrifice layer 1002; thus, a part of the structural layer 1004, which is disposed above the space portion 1006, becomes movable. Note that the part of the structural layer 1004, which is disposed above the space portion 1006 is a part of the structural layer 1004, which is in contact with a top surface of the sacrifice layer 1002 in FIG. 12B.
Described here is an example of a simple structure in which the structural layer 1004 and the space portion 1006 are provided over a substrate. A base film or a bottom electrode may also be formed over the substrate. Further, each of the sacrifice layer and the structural layer may have a multi-layer structure.
As well as being an important step in manufacturing a micromachine, the sacrifice layer etching is a rate-controlling point with many problems. For example, there is a problem in that the structural layer 1004 buckles as shown in FIG. 12D after the sacrifice layer etching, to stick to the substrate.
Further, there is a problem in that the structural layer 1004 buckles during operation, to stick to the substrate because of static electricity or the like.
In order to solve the above problems, a method of etching a sacrifice layer, or materials or shapes of a structural layer and a layer which faces the structural layer have been considered (e.g., Patent Document 1: Japanese Published Patent Application No. Hei10-107339, and Patent Document 2: Japanese Published Patent Application No. Hei11-340477).
In Patent Document 1, a method in which a sticking phenomenon of a silicon microstructure is prevented by removing a sacrifice layer by steam phase etching is disclosed.
In Patent Document 2, an acceleration sensor in which many protrusions and recesses where a width of each recess is 0.01 μm or more and 0.1 μm or less are formed randomly in at least one surface of opposed surfaces of a fixed electrode and a movable electrode is disclosed. In addition, a method in which the protrusions and recesses in the opposed surface are formed by an anode chemical conversion treatment in or after sacrifice layer etching is disclosed.